Animation persist after completion

[TheCedarPrince]

Is your feature request related to a problem? Please explain.

I was experimenting with BackgroundAction syntax and I was thinking it might be a good idea to extend SubAction syntax to BackgroundAction's. Especially for the functions draw_grid and zero_lines.

If that is the case, it might make sense to have BackgroundAction only have access to functions like draw_grid and zero_lines and not as a general Action.

Describe the solution you'd like

What I would love to see as a user interface would be something like this:

              BackgroundAction(1:180, ground), subactions = [
                 SubAction(1:60, draw_grid()),                                                    
                 SubAction(1:60, zero_lines())
                 ]

Also, I am not sure if this should be a separate issue, but I would like to see the grid lines persist after their initial animation such that I could draw on top of them later.

Describe alternatives you've considered

Currently, this would be the way to invoke this functionality:

               BackgroundAction(1:180, ground),                                        
               Action(1:60, draw_grid()),                                                    
               Action(1:60, zero_lines()),

[Wikunia]

Yeah in the latter case the lines would not be persistent.
I was actually thinking about having something like

 BackgroundAction(1:180, ground),      
Action(1:180, draw_grid; subactions=[
    SubAction(1:60, appear(:lr, :tb))
])

where appear can take some arguments like here :lr left -> right and :tb for top -> bottom and then this gets saved in the parent Action options or something such that the draw_grid function knows how to handle that :D
Then Action(1:60, draw_grid) might simply draw the grid for that many frames without any animation?

[Wikunia]

Also see #100

[Wikunia]

I think I would handle this inside the draw grid one #38 and move this out of this milestone? What do you think about it @TheCedarPrince ?